Liquid fuels generally do not burn as liquids but instead must first vaporize to a gas and mix with oxygen in order to sustain combustion. Accordingly, a liquid fuel must first be dispersed into air as fine droplets in order to provide a large surface area for evaporation and to promote intimate mixing with the oxygen in the air. The combustion or evaporation time of a 100 micron droplet, for example, is about 10 milliseconds. In contrast, a 10 micron droplet would evaporate completely in 1 millisecond, which is more desirable. Radiant heat transfer from the burning vapor helps to heat the droplets so that further evaporation occurs.
In order to provide the liquid fuel in the form of fine droplets, it is necessary for the fuels to be atomized. Liquid fuels are generally atomized by spraying the fuel into a combustion zone by various common atomization methods: 1) airblast atomizers, where a large volume of low-pressure air shatters a low-velocity jet or sheet of fuel into ligaments and then fine droplets; 2) airless or pressure atomizers, where pressurized fuel passes through a small orifice at high velocity into quiescent air to form a liquid jet, hollow cone, or sheet of fuel that breaks up into droplets from shear with the air, which normally produces larger droplet size than in airblast atomization; and 3) air-assist atomizers, where atomization is caused by both fuel pressurization and a low volume of high-velocity air and which may be considered a combination of (1) and (2) above. Atomization processes are discussed in Lefebvre, A. H., 1989, Atomization and Liquid Sprays, Hemisphere Publishing Company, N.Y.
All of these atomization methods require that the liquid fuel possess a low enough viscosity so that good atomization may occur to produce the fine droplet sizes needed for good vaporization which, in turn, produces good combustion. If the fuel viscosity is too high, atomization is poor, at best, resulting in larger than desired droplets having much less surface area. This produces poor and/or incomplete combustion.
In Beer, J. M., and Chigier, N. A., 1972, Combustion Aerodynamics, Applied Science Publishers, Limited, London, Chapter 6 entitled "Droplets and Sprays", it is noted that most practical liquid fuel sprays have a size distribution over a wide range of droplet sizes with a mean droplet size between about 75 to about 130 microns, with a maximum droplet size being preferably under 250 microns. Beer and Chigier disclose that the smallest droplets vaporize completely, but that in larger droplets formed from heavier fuels, that is, fuels having a high viscosity, liquid phase cracking occurs, which leads to the undesirable formation of carbonaceous residue, often in the form of a cenosphere.
For distillate fuels of moderate viscosity, such as about 30 centipoise at room temperature, simple pressure atomization with a spray nozzle at a pressure of about 100 to 150 pounds per square inch (psi) produces a droplet diameter distribution that ranges from about 10 to about 150 microns, with a midrange average of about 80 microns. With decreasing fuel pressure, atomization becomes progressively less satisfactory. Much higher pressures are often used to produce a higher velocity of the liquid fuel relative to the surrounding air, thereby producing smaller droplets and evaporation times.
However, conventional spray nozzles are relatively ineffective for atomizing fuels of high viscosity, such as No. 6 fuel oil, residual oil (Bunker C), and other viscous low-quality fuels. In order to transfer and pump No. 6 fuel oil, it must usually be heated to about 100.degree. C., at which temperature its viscosity is still typically at least about 40 centipoise. Atomization of such fuels is often accomplished, or at least assisted, by atomizing air pumped at high velocity through adjacent passages in or around the liquid injection ports. Much of the relative velocity required to shear the liquid and form droplets is thus provided by the atomizing air; its mass flow is usually comparable with the fuel flow and thus comprises only a small fraction of the stoichiometric combustion air.
Accordingly, there is a need to have an improved method of atomizing liquid fuels so as to accomplish at least two objectives, namely, to facilitate the effective and economical use of higher viscosity fuels and, moreover, to obtain a more favorable droplet size and size distribution to provide more complete combustion and less by-product formation, not only in such higher viscosity fuels but also in moderate viscosity and low viscosity fuels as well.
Indeed, what is most desirable is a spray having a relatively narrow droplet size distribution with an average droplet diameter in the region of from about 10 to about 50 microns or lower so that the ratio of surface to volume of the burning droplet is the largest possible, thereby causing it to receive more heat and consequently burn faster. With droplets in this size range, nearly instantaneous evaporation occurs, even with many of the higher boiling fuel species present, which results in the substantial formation of a combustible vapor (gaseous) spray, wherein the vaporized fuel and oxygen are quickly mixed in stoichiometric quantities so that burning occurs rapidly and with only a small fraction of the droplets undergoing pyrolysis. This minimizes the formation of undesirable carbonaceous particles which would otherwise adhere to furnace surfaces and/or escape the combustion chamber into the environment unless additional means are taken to prevent such occurrence.